It's probably safe to assume by looking at the writing staff and reading the comments from this site that more than a few Jalopniks are driving some slightly used vehicles of various vintages in various forms of disrepair. It's in this spirit that we take a deeper look at how emissions laws work, why the exist and what exemptions your fuel-leaking, smoke-spewing vintage DeSoto might qualify for.

Where do they come from?

Our modern car emissions regulations come courtesy of the 1990 Clean Air Act and all of its happy amendments. Like most laws affecting cars, the federal government sets up some requirements and the individual states are supposed to enforce them. These guidelines are called State Implementation Plans (SIPs) and cover everything from your neighbor's Hyundai to how your local dry cleaner gets rid of their cleaning solvents.

Unlike some of the other laws we've talked about, there's a provision for federal control if state's plans for implementing restrictions don't meet the approval of the EPA (which also depends on who is controlling the EPA, which depends on the administration which is partially related to why some states are not happy).

It's not important to understand the history and full impact of the Clean Air Act, but it is helpful to note that because vehicles sometimes travel from one state to another (especially pollution causing large trucks) there are sometimes regional commissions on air quality. Even within states the are sometimes serious variations from one county to the next in terms of what is and is not required.

How do emissions laws vary?

It would break our server to list every variation in every emissions law for every state and county and zip code, but there are some basic provisions that should be understood and are generally applicable to most states (With 50+ SIPs out there not all of this will apply to everyone). We'll draw heavily from California, which has the strictest air quality standards of any state.

Do you need to get your car checked?

If you've purchased a new car it should already come cleared for a period of 2-6 years depending on where you live and what year you purchased it. When you register your car with the state one of the fees you usually pay on a new car is an emissions testing "abatement fee" where you pay to have your car not tested.

Beyond that point you'll almost always be informed either by a color-coded sticker on your license plate or windshield (as in Texas). If you're in a state like Illinois, you'll receive a letter informing you of where you can get your car inspected and by what date this inspection has to be completed. Always double check when you move and have your car reregistered.

When buying a used car, check to see when the latest emissions test was and assure that all proper paper work has been filled out. In California, for instance, you have to pay a small fee and transfer the smog certificate. If you transfer the car within your family you're exempt from this. If you sell your car to your family, you shouldn't overcharge them.

The test

There are two predominant forms of testing, though there are even variations within the two major types. Both tests involve sniffing your exhaust to see the presence of certain chemicals (specifically hydrocarbons, oxides of nitrogen and carbon monoxide). The hope is that this test will show how efficiently your engine is burning fuel (or not burning fuel).

The basic test involves idling your car and smelling the gas with a device hooked up to a computer. The more complicated test involves a chassis dynometer. This test is referred to as an IM240 because they run your car for approximately 240 seconds to simulate a short drive with an average speed of 29.4 mph and a top speed of 56.7 mph (see the simulation graph here).

Even if your car isn't brand new, as long as you take care of it and there are no known major issues with the engine then you shouldn't have major problems. If your car has had problems or is throwing up a check engine light you might want to figure that out before you take it to the testing facility. One faulty spark plug could be the difference between PASS and FAIL. If you bought a Project Car Hell car you may want to skip down to the exemptions section.

What if I fail?

Most states give you around a month to figure out what's wrong with your car and try again. It's best to take your test results to a friend or trusted mechanic that knows the score if you're not mechanically inclined. If you are mechanically inclined, we recommend checking out this article by PopMechanics.

If you decide not to go back and get your car reinspected you could be up for many levels of trouble. Until recently, you could go to jail in Illinois for not having an up-to-date emissions test. This was because the law required a suspension of drivers license immediately upon getting caught, meaning that if you drove the car you were driving without a license which is an infraction that requires arrest.

Am I exempt?

If your car is of a certain age and meets certain requirements for being a "classic" you might be exempt. For instance, Arizona adjusted their SIP a year ago to exempt cars that were 15 years or older and could be considered a classic. If you've got a malfunctioning Sephia, you're probably out of luck because that car would never qualify as a classic in any part of the country. In California the car has to be from 1975 or earlier to be exempt.

"Testing old cars adds a layer of complexity and often cost for the state agency charged with conducting the tests and the actual benefit to air quality isn't there to justify it," said an individual involved in the process. "Cars older than 25 years are such a minute proportion of total vehicles on the road that you really can't make a difference in air quality no matter what you do with them."

The best resource is your local Department of Motor Vehicles and you can find a list of your local DMVs here. It's a much easier way of getting the information than trying to parse the many amendments to SIPs (just trust us). The website DMV.org also has simple-to-understand instructions based on the latest laws.